1. Field of the Invention
This invention relates to an improved film-wrapped blasting cartridge, particularly a chub cartridge containing a water-bearing blasting agent.
2. Description of the Prior Art
Semi-solid colloidal dispersions of water-bearing blasting agents, e.g., water gels or slurry explosives or emulsion-type blasting agents, currently are available in the form of small-diameter, i.e., less than 1.75-inch (44.5-mm), cartridges for use in underground blasting operations. The cartridge, often referred to as a "chub" cartridge, is a tube of plastic film, filled with blasting agent, and gathered at both ends and closed, e.g., by means of metal closure bands around the gathered portions.
The film wrap of a chub cartridge has to have sufficient tear and abrasion resistance that the cartridge will not fail catastrophically when subjected to the force used to load it into a borehole, especially a rough or gravelly borehole. Also, the tear resistance of the film should be sufficient that a tear made in the film for insertion of a blasting cap in a primer cartridge will not propagate to a degree such as to cause the cartridge to fail, and a significant amount of the cartridge contents to escape, before the cartridge has been positioned in the borehole. The impact strength of the film wrap also has to be high enough to insure against failure of the cartridge and loss of its contents should it be subjected to rough handling conditions prior to loading. In addition, the film should have sufficient dimensional stability to prevent gross deformation of the cartridge and attendant loading difficulties, but preferably should not be so stiff that the rigid rounded cartridge ends cause the cartridges to override and jam during stringloading.
A still further requirement on the cartridge is that it be tampable, generally understood to mean that the cartridge should fail in the borehole when subjected to the steady hard push or impact of a tamping rod in a manner such that the blasting agent will be able to escape from the cartridge and fill the borehole more readily.
Heretofore, the wrapping film for chub cartridges has in most cases been polyethylene, polyethylene terephthalate, or a combination of the two. Polyethylene has such poor dimensional stability, however, that the cartridges are difficult to load into the borehole, and difficult to tamp as well. Polyethylene terephthalate cartridges have good dimensional stability and are tampable, but have poor tear and abrasion resistance and tend to override on string-loading. A laminated film of a layer of polyethylene terephthalate bonded on one or both of its sides to a layer of polyethylene combines the dimensional stability afforded by the terephthalate and the abrasion resistance afforded by the polyethylene.
U.S. Pat. No. 3,921,529, issued Nov. 25, 1975, to Canadian Industries Ltd., describes a stiff film-wrapped explosive cartridge in which the film consists of a stiffener (unpliable) layer, e.g., regenerated cellulose, cellulose acetate, polyester, paper, or polypropylene, bonded on one or both of its sides to a resilient, pliable film, e.g., polyethylene. The cartridge is adapted to expand circumferentially in a borehole upon application of end pressure with a tamping rod by virtue of one or more longitudinal areas of weakness in the stiffener layer, which is on the inside of the cartridge, preferably sandwiched between two layers of the pliable film. Rupture of the inner stiffener layer is said to permit further stretching of the outer pliable film allowing the circumferential expansion of the cartridge, thereby in effect converting the cartridge wrap to a dimensionally unstable all-polyethylene wrap. While this cartridge has dimensional stability during loading, as well as good abrasion resistance, the tear resistance of the cartridge when punctured by a blasting cap is inadequate. Furthermore, inasmuch as the circumferential expansion of the cartridge is apt to be followed by contraction when the end-pressure is removed, the possibility of achieving a filled borehole diameter with this cartridge is uncertain. Also, with respect to cartridge manufacturing techniques, the film wrap is continuously fed into a tube-forming device from a roll. Because cuts have to be made in the inner layer of the preferred three-layered laminate, the stiffener film has to be cut prior to the preparation of the laminate, and it is not possible to make adjustments in the location or depth of the cuts in the already prepared laminate to control package quality.